A series of ultrastructural, genetical and biochemical studies of Drosophila oogenesis and embryogenesis will be conducted in order to provide an understanding of processes of determination and other developmental processes. Four specific areas will be investigated: 1. Pole cells: ultrastructural analysis of a) fate of those pole cells which do not become germ cells, b) effect of UV-irradiation on polar granules. 2. Scanning and transmission electron microscopy of wild-type development, of mutations which affect cellular determination at the blastoderm stage, and of mutations which affect anterior polarity and head involution. 3. Role of centrioles in the establishment of the endomitotic cycle of cellular growth. 4. Ultrastructural and biochemical studies of the formation of yolk protein during oogenesis and its ultilization during embryogenesis.